The present invention relates to intervertebral stabilization devices intended for maintaining at least two vertebrae, whose common disc is worn, in suitable relative position.
It is known that, in the course of ageing, the intervertebral discs risk wearing, with the result that the movements of the intervertebral articulation change, becoming abnormally more ample. The vertebrae may then move in excessive manner with respect to one another, causing permanent displacements as the vertebrae are badly positioned.
The intervertebral disc behaves more like a distributor of pressure or a three-dimensional coupler of movements than as a simple absorber of longitudinal loads.
The intervertebral movement is guided by the set of posterior articulations. The latter have only one degree of freedom; the two surfaces being able only to slide on each other.
During this movement, the disc deforms elastically, progressively braking the movement and dampening it completely at the end of amplitude.
The disc presents a visco-elastic quality. It progressively adapts by viscosity to a new anatomical relation. Consequently, the return movement is then taken over by a similar progressive damper, and this rapidly from its origin.
The set of the articular facets may be asymmetrical and consequently may create a three-dimensional movement coupling lateral bending and horizontal rotation.
This complex movement is itself coupled to the large movement of bending-extension. In complete bending as in complete extension, the amplitude of the combined movement (bending-rotation) becomes zero while it obtains a maximum amplitude in anatomical position, i.e. in the natural anatomical relation of the vertebrae when one is standing or when one is walking (in fact, in an intermediate position between bending and maximum extension).
The biological degradation of the disc disturbs these mechanics of coupled and dampened movements.
This evolution is a source of discomfort and of pain.
U.S. Pat. No. 4 743 260, for example, already proposes placing between at least two adjacent vertebrae a flexible stabilization device composed of two elements fixed to the vertebrae in question. The stabilization elements are made of a non-metallic material which is resistant but sufficiently flexible to allow at least a normal movement of the backbone.
The stabilization elements in question are made of a carbon-fiber-reinforced plastic material, with the result that they have a certain flexibility. However, this flexibility is limited to very small amplitudes in the sense of their curvatures, but no elasticity in traction or in compression.
It has thus been proposed to place a supple, non-elastic tie between two vertebrae to limit like a sudden brake, the movement in its amplitude in bending. At that stage, it behaves like a rigid system, transferring the mechanical stresses on the adjacent intervertebral articulations.
This comes back to the complications of mechanical overload as for arthrodeses or rigid metal assemblies.
Furthermore, the degenerative pathological movement between two vertebrae is not only a movement of exaggerated amplitude but also a subtle disorganzation of the three-dimensional coupling of different degrees of freedom.
A simple supple tie can but help, only insofar as it positions the two vertebrae in an extreme amplitude and therefore in extension.
The residual micro-movements possible due to the creeping of the tie are capable of contributing an elementary, rough adaptation with respect to functional needs. However, their existence clearly differentiates this fixation with respect to the rigid system (arthrodesis or metallic fixation).
It is an object of the improvements forming the subject matter of the present invention to produce devices capable of accompanying the ageing of the discs and which therefore constitute prostheses avoiding the drawbacks mentioned hereinabove.
It will be understood that the purpose of the proposed system is to make good in the most physiologically possible the shortcomings of the disc created by the biological and pathological conditions.
The system proposed aims at obtaining a new stability from an intervertebral position approaching the neutral position (between bending-extension).
In this position, the possibility of movements of the posterior articular surfaces is greater in particular in the sense of extension.
Furthermore, the asymmetrical work of the two posterior surfaces is possible.
The stability of the intervertebral articulation is then obtained due to the quality of damper of the device according to the invention.
The role of the invention is to accompany the movement of the articulation by limiting it slightly in bending and by avoiding abnormal displacements. Operating in parallel, while maintaining the vertebrae in extension with respect to one another, the invention avoids too much contact of the articular surfaces on one another. The system according to the invention also limits the closure of the lateral recess, consequently preventing the possible compression of the nerve root.